An active-matrix-type display device typically includes a TFT device that has a plurality of thin film transistor (TFT) elements, for achieving driving and light emission in units of sub-pixels. In connection with this, recently, TFT devices with a channel layer made of an oxide semiconductor are in development, for use in display devices and the like (refer to Patent Literature 1). The following describes a conventional TFT device with a channel layer made of an oxide semiconductor, with reference to FIG. 23.
The conventional TFT device illustrated in FIG. 23 includes a substrate 1900. Further, the conventional TFT device includes, disposed above the substrate one on top of another in the stated order: a gate electrode 1901; a gate insulating film 1903; a channel layer 1904; a protection film 1912; a drain electrode 1907; and a source electrode 1908. The channel layer 1904 is a layer of an oxide semiconductor including at least one of In, Zn, and Sn.
The protection film 1912 is made of an amorphous oxide semiconductor (e.g., amorphous SiO), and is in contact with the channel layer 1904. Further, the protection layer 1912 in this conventional TFT device is defined such that, when thermal desorption measurement is performed with respect to the protection layer 1912, oxygen of at least 3.8×1019 cm−3 is observed as desorption gas.
The conventional TFT device illustrated in FIG. 23, due to including the protection film 1912 whose film characteristic is defined as such, suppresses the generation of oxygen defects in the channel layer 1904 made of an oxide semiconductor, and thus suppresses an increase in off current deriving from generation of a large number of carrier electrons that would otherwise occur. Thus, this conventional TFT device has desirable characteristics with an excellent on/off ratio.